I used to have one, but its not very useful for freediving. When you descend, it starts timing the dive. When you surface, it stops. But then when you descend again, it continues timing from where it left off, considering it all one scuba dive. You have to do the math in your head to get the duration of all but the first dive.
It does show you your depth and the temperature, but it doesn't do surface interval, and that is one of the best things about my D3.
I couldn't remember why I trashed the idea to get one of those watches until you posted this. Thanks for pointing the surface time problem. I think they require something like 10 or 20 minutes between dives to count them as "new dives" so they would likely not be the best choices for spearing or freediving (obviously).